Thermoplastic blends of poly(arylene ether) resins and polystyrene resins are currently produced in large volumes and are highly valued for their balance of properties including stiffness, impact strength, tensile strength, and heat resistance. Some properties of the blend can be superior to those of either component resin alone. See, for example, U.S. Pat. No. 3,383,435 to Cizek, which illustrates blend flexural strength, flexural modulus, compressive strength, tensile strength, impact strength, and hardness values that are superior to corresponding values for the component resins.
Many possible product applications for thermoplastic resins require that the resin be free of any objectionable odors. Considerable effort has been expended to reduce odor components associated with poly(arylene ether) resins. Poly(arylene ether) resins are typically synthesized in the presence of odoriferous organic amines, and the poly(arylene ether) resin may incorporate and later liberate such amines. Thus, one effort to reduce the odor of poly(arylene ether) resins has focused on the removal of volatile components during extrusion. For example, U.S. Pat. No. 3,633,880 to Newmark describes an extruder that includes elements to alternately compress and decompress the resin, thereby liberating volatile components, and a plurality of vacuum vents to remove the volatile components. As another example, U.S. Pat. No. 4,746,482 to Ribbing et al. describes an extrusion process whereby a polyphenylene ether resin is melt kneaded under vacuum prior to mixing with another resin.
Another source of odor in poly(arylene ether) resins is impurities in the phenol monomer, which is oxidatively polymerized to produce the poly(arylene ether). Odoriferous impurities in the 2,6-dimethylphenol monomer, such as 2,4,6-trimethylanisole, may be substantially reduced using particular distillation procedures as described in U.S. Patent Application Publication No. US 2004-0211657 A1 to Ingelbrecht. Alternatively, the build-up of such impurities in the recycled solvent of a poly(arylene ether) process may be reduced by solvent purification methods described in U.S. Pat. No. 4,906,700 to Banevicius.
Yet another approach to reducing the odor of poly(arylene ether) resins and their blends with polystyrene has been to add materials to the extrusion process that reduce the odor of the extruded resin. For example, U.S. Pat. No. 5,017,656 to Bopp describes the addition of carboxylic acids and/or acid anhydrides during extrusion. In that reference, extruded pellets were subjectively graded for their odor, but no chemical analyses of specific odor components were reported. As another example, European Patent No. 480,244 B1 to Bopp et al. describes addition of carboxylic acids or anhydrides during extrusion, combined with “steam stripping” of the resin (i.e., addition of water to the extruder and venting of the resulting water vapor). That reference was particularly concerned with reducing the level of butanal impurity in the resin. As yet another example, U.S. Pat. No. 4,369,278 to Kasahara et al. describes extruding blends of polyphenylene ether and rubber reinforced polystyrene in an extruder with a vacuum vent, and optionally adding a pyrolysis inhibitor (e.g., a hindered phenol or a phosphite compound) and/or water to the extruder. The working examples of the Kasahara et al. patent indicate that the total content of volatiles in the resin blend may be reduced to levels as low as 2500 parts per million.
In order to serve aesthetically demanding markets such as panels for automobile interiors, there remains a need for methods of further reducing the level of volatiles in blends of poly(arylene ether) and polystyrene resins. In particular, there is a need for methods of compounding these resins that reduce the level of styrene in the resulting blend.